There's a post called "LX Plotting Structure" in the Plays & Musicals forum that talks a bit about using light walkers.
I'll repost my light walker words from that discussion here:
In many situations where the actors are union (ie, would need to be paid for the hours they spend at the theatre during lighting work), theatres bring in volunteer "light walkers" who just move around the stage in the places the actors would be while the LD, SM, and director do their lighting plot. It is very tedious for the light walkers because they are totally passive and don't know where they're going or what they're doing - they just walk left when you tell them to walk left. But if you can find nice, patient people who are excited about being onstage under lights (however un-glamorous the situation), it can be a pretty painless way to get lighting work done. In this situation, you can get your basic lighting done without tiring the actors, and the LD can refine lighting for specific moments with the real actors during tech rehearsals.
Do be sure to warn the light walkers that they should not wear black or white, and make sure they know that they could be standing on their feet for hours at a time. In these tech sessions, by the way, the PSM is usually at the ready to talk through the blocking with the light walker, but you sit next to the director (who decides what scenes to move to) and the LD (who talks through calling the cues with you) so that you don't need to actually be on the stage. We usually give the light walkers a couple of comp tickets and a big thank you.